In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile stations and/or user equipment units communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-comprised, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “NodeB” or “B node” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Long Term Evolution (LTE) together with Evolved Packet Core (EPC) is the newest addition to the 3GPP family.
Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. Mobility procedures between LTE and CDMA systems are defined by 3GPP. These procedures are to a large extent defined as tunnelling of CDMA messages from the user equipment to the CDMA system through the LTE system. These mobility procedures are, for example, SRVCC to 1×RTT CDMA, CSFB to 1×RTT, PS HO to CDMA HRPD for PS, and eCSFB (e.g., CSFB with simultaneous PS HO to CDMA).
These functions support voice applications and data applications. CSFB supports voice when IMS voice applications are not implemented in LTE, while the SRVCC implies handoff of an on-going IMS voice call from LTE to 1×RTT CDMA. The PS HO to HRPD implements the handoff of a data session from LTE to CDMA. The eCSFB is specified such that simultaneous voice fall back and data handoff can be done for data session continuity while taking a voice call.
The mobility procedures include optimized as well as non-optimized hand offs. The optimized solutions depend on signaling between EPC and CDMA Core to prepare for seamless handoff, while the non-optimized solutions depend on the user equipment re-selecting to a CDMA cell when needed. The optimized mobility solutions rely on specified interfaces between the systems. The S102 interface between the 3GPP MME and the 1×RTT MSC Server supports CSFB or SRVCC to 1×RTT, as shown in FIG. 1. Paging, CSFB or SRVCC messages are tunneled over S102. The S101 and S103 interfaces between the 3GPP MME and the HRPD (packet node) supports PS HO signaling from LTE to CDMA and S103 is a user data interface for data forwarding while doing PS HO, as shown in FIG. 2.